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United States Patent |
5,318,125
|
Wittrisch
|
June 7, 1994
|
Method for continuing measurements after recovery of a measuring tool
immobilized in a well
Abstract
A method for continuing a measuring operation using a sonde immobilized in
the well which method involves lowering, concentric to the cable, a length
of tubular elements until the sonde is engaged by a special coupling
fitted at the end of the length of tubular elements, the length of tubular
elements serving to protect the cable. In addition, a coupling at an upper
end of the length of tubular elements is equipped with a lateral window to
minimize maneuvering time. After engagement, the sonde is used to carry
out measurements by displacing the length of tubular elements.
Inventors:
|
Wittrisch; Christian (Rueil Malmaison, FR)
|
Assignee:
|
Institut Francais du Petrole (Rueil Malmaison, FR)
|
Appl. No.:
|
988121 |
Filed:
|
April 8, 1993 |
PCT Filed:
|
June 10, 1992
|
PCT NO:
|
PCT/FR92/00523
|
371 Date:
|
April 8, 1993
|
102(e) Date:
|
April 8, 1993
|
PCT PUB.NO.:
|
WO92/22728 |
PCT PUB. Date:
|
December 23, 1992 |
Foreign Application Priority Data
Current U.S. Class: |
166/277; 166/50; 166/250.01; 166/301 |
Intern'l Class: |
E21B 047/00; E21B 031/00 |
Field of Search: |
166/277,301,250,50
|
References Cited
U.S. Patent Documents
4023847 | May., 1977 | Webb | 294/86.
|
4597440 | Jul., 1986 | Pottier | 166/250.
|
4678038 | Jul., 1987 | Rankin | 166/301.
|
4945987 | Aug., 1990 | Wittrisch | 166/250.
|
5042297 | Aug., 1991 | Lessi | 166/250.
|
5111880 | May., 1992 | Wittrisch et al. | 166/250.
|
5180011 | Jan., 1993 | Wittrisch et al. | 166/250.
|
5217072 | Jun., 1993 | Wittrisch | 166/250.
|
Foreign Patent Documents |
2609105 | Jul., 1988 | FR.
| |
2094865 | Sep., 1982 | GB.
| |
Primary Examiner: Dang; Hoang C.
Attorney, Agent or Firm: Antonelli, Terry, Stout & Kraus
Claims
I claim:
1. A method for controlling measurements by means of a measuring sonde
immobilized in a well, said sonde being connected to the surface by a
cable comprising at least one conductor electrically connecting said sonde
to a surface control installation, and said cable being operated by means
of a winch, said method comprising the following stages:
cutting the cable above the level of a rotary table of a derrick floor,
fixing a half-connector onto each of the two ends of the cut cable, said
half-connectors being adapted to constitute a quick coupling for
assembling said cable,
lowering into the well a tubular string for grappling the immobilized
sonde, said string containing in its inner channel a lower length of the
cable substantially taut between said sonde and the derrick floor, said
string comprising at least one grappling sub adapted for grappling said
sonde and a determined length of maneuvering tubular elements,
fixing a side-entry sub onto the upper end of said determined length of
tubular elements, said sub comprising a side wall passage through which
said lower length of cable passes from the inside towards the outside of
the tubular elements, connecting outside the string the two ends of the
cable and connecting said conductors of said cable,
adding above said sub a length of tubular elements adapted to reach the
sonde immobilized in the well, while keeping said cable substantially
taut,
guiding the string by means of said lower length of cable so as to grapple
the sonde by way of said grappling sub,
carrying out measurements of servicings with said sonde fastened through
said grappling sub to the lower part of said string and connected to the
surface through said cable.
2. A method as claimed in claim 1, further comprising:
selecting the determined length of tubulars contained between the grappling
sub and said side-entry sub substantially equal to a length of mechanical
protection of said cable adapted both to reach the sonde with said
grappling sub without damaging the cable and for displacements of the
sonde during continuation of the measurements.
3. A method as claimed in claim 1 or claim 2, comprising;
carrying out measurements with said grappled sonde while going deeper than
the point of immobilization of said sonde by adding tubular elements to an
upper part of the string while keeping said cable substantially taut.
4. A method as claimed in claim 1 or claim 2, comprising:
carrying out measurements with said grappled sonde by going higher than the
point of immobilization, by disassembling at most the length of tubular
elements located above said side-entry sub while keeping said cable
substantially taut.
5. A method as claimed in claim 1, further comprising;
circulating the drilling fluid by pumping through the grappling string, the
side-entry sub comprising seal means around the passage of the cable
between the inside and the outside of said string.
6. A method as claimed in claim 1, further comprising:
detecting the grappling of the sonde by said grappling sub by means of the
surface control installation connected to the sonde by said conductors of
said cable.
7. A method as claimed in claim 1, further comprising:
using a quick mechanical coupling to connect said cable, comprising means
for connecting electric conductors of the cable.
8. A method as claimed in claim 1, further comprising:
breaking the cable at a the brittle point located at the top of the sonde,
taking up the cable by means of the winch and taking the sonde up to the
surface by the operation of pull-out of the string.
9. Application of the method as claimed in claim 1 or claim 2 to an oil
well deflected or not with respect to the vertical and in which a
measuring or servicing sonde, connected to the surface by a cable
comprising at least one electric conductor, is immobilized, and said sonde
cannot reach the measuring or servicing zones of said oil well by action
on the cable.
10. Application as claimed in claim 9, wherein the sonde is stuck in said
well.
11. Application as claimed in claim 9, wherein the sonde cannot reach the
measuring or servicing zones because of the too strong inclination of the
well with respect to the vertical, which does not allow cable descent of
said sonde by gravity.
Description
FIELD OF THE INVENTION
The present invention relates to a method for continuing measurements which
have been interrupted or which have not started yet, when the measuring
sonde is immobilized in a well into which it has been lowered at the end
of a maneuvering and measure transmission cable.
BACKGROUND OF THE INVENTION
There are several well-known methods for trying to grapple a measuring
sonde whose handling is no longer possible through the cable, but none
allows measurements to be continued at the place where they have been
stopped following the immobilization of the sonde or its sticking.
A first method is common to all operations for fishing tubular parts stuck
or lost in a wellbore. The well is first cleared of the measuring cable so
that it does not hinder later operations. To that effect, the cable is
pulled until it breaks at the brittle point which is located on the fixing
device on the upper part of the sonde. After taking up the cable by means
of its winch, a fishing string mainly consisting of an "overshot", as it
is commonly called in the profession, adapted to clutch the top of the
sonde, is run down into the well. The other components are conventionally
pipes and drill collars. In this method, the difficulty consists of
covering the sonde with the overshot in the absence of guiding and while
groping along from the surface. This operation may actually only succeed
at a slight depth and in instances where the drill hole is well calibrated
and where the axis of the tubular to be fished is almost parallel to the
axis of the well. Concerning measuring sondes, most of them have a small
diameter with respect to the hole and these conditions are scarcely
present, except in small-diameter boreholes.
The most common method is called the "cut and thread" method. It consists
of cutting the cable at the level of the derrick floor without dropping
the part of the cable linked with the sonde into the well. Thus, the two
ends of the cut cable are provided with two half-elements constituting a
quick coupling. Assemblage of the overshot and of the first drill collars
is started in the derrick. The end of the cable connected to the winch is
passed through these first elements when they hang on the pipe hook. The
two ends of the cable are connected through the quick coupling. The cable
may then be maintained taut by its winch while the overshot and the first
pipes are lowered around the latter into the well. After hanging them onto
the rotary table, the cable is maintained before the quick coupling is
opened so as to pass the end of the cable connected to the winch through
new tubular elements assembled and hung on the pipe hook, as previously.
The lowering maneuver is continued by repeating this operation until the
overshot, guided by the coaxial cable, covers and clutches the top of the
sonde. The fishing operation is ended, as in the previous method, after
the cable has been broken.
This operation is long because passing the cable through each assembled
length of tubular elements causes a waste of time which is relatively
considerable in relation to the usual maneuver time. In case of sticking
in a borehole, it is an accepted fact that speed is a preponderant factor
for the success of the fishing operation.
None of the two methods described above allows measurements to be achieved
or continued with the sonde in said well.
The method in accordance with the present invention reduces the maneuver
time by limiting the number of operations for running the cable through
the length of tubular elements, by using advantageously a side-entry sub.
With the "cut and thread method", the sonde is never connected electrically
to the surface and it has never been attempted to keep the use of the
sensors of the sonde once the latter is stuck. In fact, the use of a quick
coupling including sealed connections for the conductors is of no interest
here since the cable will be broken after the sonde has been clutched.
Furthermore, the cable being coaxial to the string of tubular elements
over its total length, it is not possible to move the sonde while keeping
the entire cable continuity.
The method of the invention also has the advantage of allowing measurements
to be continued when the sonde has been clutched, be it towards the bottom
of the well or higher up towards the surface. The measuring operation,
which has been interrupted or made impossible by the immobilization of the
sonde, will not be totally missed since it is now possible, with the
present method, to carry out the total or at least part of the measuring
program.
Besides the main advantage cited above, the invention provides a means for
knowing precisely the moment of contact of the grappling sub with the head
of the sonde, then for checking the holding back of the sonde by said sub.
In fact, the sonde is completely operational since it is connected
mechanically and electrically to the surface installation, as at the
beginning of the operation. By means of sensors and through the
transmission of the signals towards the surface, the operator may control
that the displacement of the string makes the sonde move identically. This
advantage guarantees not only that further measurements will be possible,
but also that grappling of the sonde will succeed, unlike prior methods
which provide no reliable information on the quality of the grappling of
the sonde, which accounts for the relatively high failure rate in the most
difficult cases.
Another method called "side door" method may also illustrate the prior art.
It consists of using a special overshot having a lateral opening allowing
the measuring cable to be passed outside the fishing string. The cable
needs not be cut. The string may then be lowered in a conventional way.
The overshot is guided onto the head of the sonde as in the "cut and
thread" method, then the operation is continued according to the same
methodology. This "side door" method is not used for wells deeper than
1,000 meters because it involves high risks of damage of the cable upon
lowering of the string towards the well bottom, and in case of cable
breakage, the absence of guiding of the overshot most often compromises
recovery of the sonde. In fact, when the overshot gets close to the head
of the immobilized or stuck sonde, the well will, by that fact, give rise
to considerable friction on the end of the string outside which the cable
is located and is therefore very vulnerable. Moreover, the mechanical
actions necessary to grapple the sonde are most often exceed the strength
of conventional cables. In order to make the limited use of this method
quite clear, the following recommendation, given to sonde fishing
operators, may be cited: "the side door method should not be used to fish
tools in open holes, but rather to fish tools stuck at the shoe of a
casing string". But when measuring tools are at this level, measurements
are generally finished.
SUMMARY OF THE INVENTION
The method of the present invention allows operations in deep, difficult,
deflected wells, and also in open holes, because the cable is only present
in the annulus defined by the tubular elements and the well at a depth
chosen by the operator, where he knows that the cable does not risk any
damage. The cable is thus protected against outer friction over a
determined length with the method. The protection corresponds to the
length of the string between the grappling sub and the side-entry sub.
According to its claims, the present invention thus provides a method
allowing measurements to be continued by means of a measuring sonde
immobilized in a well, said sonde being connected to the surface by a
cable comprising at least one conductor connecting electrically said sonde
to a surface control installation, and said cable may be operated by means
of a winch.
The invention comprises the following stages :
cutting the cable substantially above the level of the rotary table,
fixing a half-connector on each of the two ends of the cut cable, said
half-connectors being adapted to constitute a quick coupling for
assembling the two ends of said cable,
lowering into the well a tubular string for grappling the immobilized
sonde, said string comprising, in its inner channel, a the lower length of
the cable substantially taut between said sonde and the derrick floor,
said string comprising at least one grappling sub adapted to grapple said
sonde and a determined, length of maneuvering tubulars elements,
fixing a side-entry sub onto the upper end of said determined length of
tubular elements, said sub being adapted to pass said lower cable length
from the inside to the outside of the tubular elements, connecting outside
the string the two ends of the cable and connecting said conductors of
said cable,
adding, above said sub, the corresponding length of tubular elements to
reach the sonde immobilized in the well, while keeping said cable
substantially taut,
guiding the string by means of the coaxial cable so as to grapple the sonde
by way of said grappling sub,
carrying out measurements or servicings with said sonde grappled through
said grappling sub to the lower part of said string and linked to the
surface by said cable.
The method of the invention allows makes it possible to select the
determined length of tubular elements contained between the grappling sub
and said side-entry sub substantially equal to a length of mechanical
protection of said cable, adapted both to reach the sonde with said
grappling sub and to perform displacements during the continuation of the
measurements without damaging the cable.
With the previous method, measurements may be carried out with said
grappled sonde while going deeper than the depth of immobilization of said
sonde, by adding tubulars elements to the upper part of the string.
Measurements may also be achieved with said grappled sonde while going
higher than the depth of immobilization, by disassembling at most the
length of tubulars located above said side-entry sub, while keeping said
cable substantially taut.
The method may allow circulation of the drilling fluid by pumping through
the grappling string, the side-entry sub comprising seal means around the
passage of the cable between the inside and the outside of said string.
The invention may provide a method for detecting the grappling of the sonde
through said overshot by means of the surface control installation
connected to the sonde by said conductors of said cable.
A mechanical quick coupling comprising means for connecting said electric
conductors of said cable may also be used.
The method in accordance with the invention may allow the cable to be
broken at the brittle point located at the top of the sonde and to be
taken up by means of the winch. The sonde is taken up to the surface by
the operation of pull-out of the string.
The previous method and all its variants may be used in oil wellbores,
deflected or not with respect to the vertical, in which a measuring or a
servicing sonde connected to the surface by a cable comprising at least
one electric conductor is immobilized. Said sonde cannot reach the zones
of said oil well in which measurements or servicings are performed by
action on the cable.
One particular application may be characterized in that the sonde is
immobilized by sticking in the well.
Another advantageous application may be characterized in that the sonde
cannot reach measurement or servicing zones because of the too high
inclination of the well with respect to the vertical, which does not allow
descent of said sonde by gravity.
BRIEF DESCRIPTION OF THE DRAWINGS
Other features and advantages of the present invention will be clear from
reading the description hereafter given by way of non limitative examples,
with reference to the accompanying drawings, in which :
FIGS. 1A, 1B, 1C, 1D and 1E illustrate various stages of the grappling of
the sonde with the method of the invention;
FIGS. 2A and 2B show the measurement operations according to the method of
the present invention; and
FIG. 3 shows an embodiment of a side-entry sub.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
FIG. 1A shows a well comprising a cased length 2 and an open hole length 3.
A measuring or servicing sonde is immobilized in the open-hole section at
a depth 25. The sonde has been lowered into the well by means of cable 4
operated through winch 5 located at the surface. The cable comprises
conductors which connect electrically the sonde 1 to a control
installation 6.
In the following description of the present invention, the term depth will
refer to the length of the well measured from a fixed reference point
located at the surface. It is generally the rotary table, but it may also
notably be measured from the ground or from the seafloor. The change of
the measurement reference point will have no effect on the description and
the scope of the invention.
Also, without departing from the scope of this invention, the sonde may be
immobilized at a depth where the well is cased, and similarly, the well
may have no cased length yet.
In the invention, the sonde operated through cable 4 may be stuck
mechanically in the well in such a way that it cannot be taken up to the
surface or down towards the well bottom. Without departing from the scope
of the invention, the sonde may be prevented from being displaced in only
one direction, be it towards the surface or towards the bottom. This may
be due to a partial mechanical sticking or to the fact that the well
inclination is such that the action of gravity is no longer sufficient to
allow descent of the sonde hanging on the end of cable 4. In this case,
the sonde is immobilized when friction on the sonde becomes stronger than
the force of gravity acting on the sonde. The immobilization depth may
then be either the depth from which the sonde can no longer go down
towards the well bottom, or a lower depth located above the latter,
because the operator preferably chooses, in this case, to set the method
of the invention into action with a sonde which is not laid on the walls
of the well, but which hangs on the cable. To that effect, he pulls on the
cable so as to take the sonde up to a determined depth.
In all the cases cited previously, the section of cable 4 connected to
sonde 1 is supported by a conventional jaw device 9 set at the level of a
rotary table 8 of the derrick floor. The cable is cut substantially above
table 8 and two half-connectors 7 are fastened onto each end. The quick
coupling constituted by the two half-connectors is of a conventional type
comparable to those used for the "cut and thread" method. Without
departing from the scope of this invention, a specific quick coupling
comprising means for connecting cable conductors may be used. This
specific coupling may be, for example, a quick plug-in socket capable of
supporting the weight of the cable while connecting electrically the sonde
to installation 6, or more simply a quick mechanical coupling which also
allows the conductors to be connected to one another. But, advantageously,
the electric connections are only achieved when indispensable, that is
when the side-entry sub is set on the string.
After the stage illustrated by FIG. 1A, the operators assemble the first
tubular elements of the grappling string above the rotary table. When the
latter are still hanging on the lifting hook, the cable end 10 connected
to the winch is then passed through these first elements, then the two
ends of the cable are linked by connecting the two half-connectors 7.
Quick coupling 14 is constituted thereby.
FIG. 1B shows the stage where the first string elements 12 hang on
elevators 30 and comprise at their end the grappling sub 11 adapted to
co-operate with the head of the immobilized measuring sonde. Quick
coupling 14 being assembled, cable 4 is tightened through an action on
winch 5. The means 9 for hanging the cable are removed. The driller lowers
elements 12 into the well while keeping cable 4, located inside elements
12, substantially taut. The driller hangs them on the rotary table with
the conventional means used in the profession. It should be noted that
cable 4 and consequently quick coupling 14 are stationary with respect to
the rotary table and that elements 12 are lowered concentrically to said
cable.
In FIG. 1C, the hanging means 9 are placed on the cable and the quick
coupling may then be disconnected.
The operators repeat the previous operations by passing the end of part 10
of the cable through another length 13 of tubular elements. After
connection of the cable, the latter is tightened again, hanger 9 is
removed and element 13 is screwed onto element 12. The assembly is lowered
into the well and hung onto the table thereafter. These operations follow
one another until the desired string length including the cable in its
inner channel is constituted. This length 16 is shown in FIG. 1D.
A side-entry sub 28 is screwed onto the upper end of the determined length
16. This device is adapted notably for three main functions:
passing a cable from the inner channel of a tubular element towards the
outside thereof,
forming a seal around the cable at the level of the window allowing the
previous function,
letting the cable free to slide in the window, at least in the direction of
sliding from the inside towards the outside, that is when the cable is
pulled by means of the winch.
Such a side-entry sub is well-known and may be illustrated notably by
documents FR-2,502,236 or U.S. Pat. No. 4,607,693.
The end of the part of cable 4 connected to the sonde is passed through the
opening of the side-entry of said sub and connected mechanically and
electrically to part 10 by means of a connector 27. This coupling restores
the electric continuity of the conductors of the cable, it has to be
drilling mud-tight and withstand a traction at least higher than the
tensile strength of the cable. Without departing from the scope of this
invention, a quick coupling 14 respecting the conditions stated for
special coupling 27 may be used.
One operational difficulty then consists of screwing the side-entry sub
when the cable is passed through the opening of the window. In fact, it is
recommended to avoid applying torsions and frictions onto the cable. This
is why it may be advantageous to use a side-entry sub device such as that
illustrated in FIG. 3.
FIG.3 shows a sub referred to as a "three-part" sub. Element 31 is the
side-entry sub proper, comprising a side entry 34 provided with a sealing
system and with a device for possibly fastening the cable. This sub is
screwed through a thread 39 onto another sub 32 comprising a screwing ring
35. This ring rotates freely around the cylindrical extension 42 of sub
32. A device 37 holds ring 35 in a fixed longitudinal position with
respect to sub 32. This device may be constituted from a circular ring in
two parts screwed radially in a groove 43 machined in extension 42. This
device will be dimensioned so as to support the weight hanging on the ring
by means of thread 38. A sealing system 41 completes the assemblage of the
ring on the extension.
The lower third sub 33 co-operates with a lower string of tubular elements
through its thread 40. An antirotation system 36 fastens sub 33 angularly
with respect to sub 32.
Mounting of this three-part sub is achieved as follows:
screwing the lower sub 33 on the top of the tubular string of tubular
elements hanging on the rotary table. The cable is kept coaxial,
elements 31 and 32 are previously screwed and locked by thread 39,
the cable being held on sub 33 by hanger 9, the free end is passed through
opening 34 and connector 27 is assembled. The weight of the cable may then
be controlled through winch 5, hanger 9 is removed,
placing assembly 31 and 32 onto sub 33 while making keying 36 coincide,
rotating ring 35 so as to screw the assembly 31 and 32 onto sub 33, without
rotating the assembly with respect to sub 33. It should be noted that the
antirotation system 36 must have a sufficient length and longitudinal play
so as to be able to interlock at the beginning of the screwing operation
and to allow the displacement corresponding to the screwing.
Determination of length 16 is important because it represents the wear
bushing of the cable between the grappling sub and the side-entry sub.
In the example shown in FIG. D, the operators consider that the cable is in
danger if it is in the annulus of the open hole, that is deeper than the
shoe 26 of casing 2.
In order to reach the sonde immobilized at a distance 17 from the shoe and
for the cable to be protected by the string in the total open-hole
section, length 16 must be at least equal to the length 17 which
corresponds to the length of the open-hole section between the shoe and
the immobilization depth.
If measurements are to be carried out deeper than the immobilization point
while keeping the cable protected in the total open-hole section, length
16 must be equal to the length of the open-hole section down to the
furthest measurement depth. If the well bottom is to be reached, length 16
must be equal to the total length of the open-hole section.
In the same instance, it is obvious that it will be possible to carry out
measurements between immobilization depth 25 and shoe 26 while keeping the
cable protected in the total open-hole section, except if the length of
casing 2 is shorter than length 17.
Without departing from the scope of this invention, the cable protection
length may be different from the length of the open-hole section between
the sonde and the shoe of the last casing. In fact, if part of the open
hole, under the shoe, is properly calibrated and stable, it may be decided
to lower the side-entry sub 28 down to this zone and thus have the cable
in the uncased annulus.
It is actually advantageous to limit the length 16 of tubular elements
passing around the cable because it is a long and tedious operation. But
the risks incurred will have to be assessed.
Conversely, if a casing exhibits sharp bends resulting from deflections
provided for example by a side tracking operation, it may then be decided
not to lower sub 28 deeper than the side track depth where sticking of the
cable through the tubulars can be foreseen. The side tracking operation
consists of plugging a well with cement at a certain depth when the
drilling operation can no longer be achieved as planned. A window is cut
out in the casing, above the plug, and the well is deflected by forming an
S-shaped trajectory. This S-shaped trajectory provides considerable
friction.
FIG. 1E shows the grappling 18 achieved by the grappling sub 11 on the head
of sonde 1. To reach this depth, the operators have assembled the length
20 of tubular tubular elements in a conventional and therefore faster way,
without being hindered by a coaxial cable. Cable 4 exhibits a length 19 in
the casing-string annulus. During the descent of length 20 of the
grappling string, cable 4 is kept taut by means of winch 5. The sonde
being still immobilized, the side-entry sub slides along the cable when
the tubular string is lowered towards sonde 1.
When they get close to the head of the sonde, operators fasten a
circulating head onto the upper part of the string so as to wash the
grappling sub through circulation in the string. As it has been mentioned
above, the side entry of sub 28 comprises a sealing system.
Gripping of the sonde is achieved through controlled tension on the cable
and through the downward motion of the grappling sub. Operators find their
way about notably by measurement of the lengths and by the reactions of
the sensors of the sonde since the latter remains operational by means of
the connections established by connector 27. Grappling may be visualized
by control installation 6.
If the sonde is stuck mechanically, it is released according to the usual
procedure while having the possibility of controlling the displacement of
the sonde.
FIG. 2A shows the descent of the sonde deeper below immobilization depth 25
by a length shown here by bracket 21. The string length 22 represents in
this case the sum of lengths 20 and 21. Measurements are carried out over
this length 21 if need be. If the length 17 of the open-hole section
between the immobilization point and shoe 26 is less than or equal to the
depth of shoe 26, measurements may also be carried out over length 17.
In all other cases, the maximum upper measurement depth is determined when
sub 28 is above ground.
If need be, it remains possible, at this stage of the method, to lower the
measuring sonde into the well again so as to complete measurements or to
carry out other servicings.
When operations are to be ended, the side-entry sub 28 being above ground,
traction is applied onto cable 4 so as to break the brittle point 24 and
the cable is entirely taken up through sub 28. When this operation is
over, the sonde is taken up to the surface by disassembling the grappling
string with the usual care.
Without departing from the scope of this invention, the well may be a
complete open hole comprising no casing.
This invention is not limited to servicings in an uncased or a partly cased
well. It is actually applicable and very advantageous when the measuring
sonde run inside the casings is immobilized notably through the
considerable friction provided by bends, deformations or deteriorations in
a zone of these casings.
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